508 Sir David Brewster 07i the Existence of Crystals 



of the crystal where there are no large ones, the explosive 

 force is not strong enough to separate the laminae. The fluid 

 is merely driven between the laminae to a small distance around 

 the cavity, and shows itself as a dark brown powdery matter, 

 encircling the cavity as the burr of a comet does its nucleus. 

 When the cohesion of the laminae is great, it resists the ex- 

 plosive force over a large cavity, and the contents of the cavity 

 are thrown to a considerable distance around it, and remains 

 between the laminas, either as a sort of powder, or as a con- 

 geries of minute crystals, which are sometimes large enough 

 to show their depolarizing action. When the laminae sepa- 

 rate, we find this crystalline matter either fluid or indurated ; 

 exhibiting, when fluid, the extraordinary properties described 

 in my former papers. If we breathe upon the indurated 

 matter it becomes fluid, recrystallizes in new spicule and cry- 

 stals; and, on several occasions, I have found fine examples 

 of circular crystallization. 



After the explosion of cavities containing only the dense 

 fluid, I have been surprised to find, and that in large cavities, 

 that no trace of matter was left upon the sides of the cavity or 

 around it. Whether this arose, as the fact seems to indicate, 

 from the dense fluid being a condensed gas, or from some 

 other cause, it will require new experiments to determine. 



In a very remarkable specimen, in which the cleavage plane 

 passed through a great number of large flat cavities, the brown 

 matter has been lodged near to the edges of each cavity, and 

 marks them out even to the unassisted eye. These cavities 

 were filled almost solely with the volatile fluid ; and since the 

 faces of the cavities are corroded as if by the action of a sol- 

 vent, developing crystalline forms, there is reason to think that 

 the fluid has exercised this action, and that the phasnomenon 

 is analogous to that external action, on the faces of hundreds 

 of Brazil topazes in my possession, which I have described in 

 the Cambridge Transactions*, and the singular optical figure 

 formed by which, I have represented in a late volume of the 

 Transactions of this Society f. 



The only chemical experiment on the contents of these 

 cavities, which I have had occasion recently to make, is per- 

 haps worth reporting. One angle of a cavity was blown off" 

 by its explosion, and though the fluids escaped, a pretty large 

 prismatic crystal remained within the cavity. I introduced 

 water and alcohol successively into the cavity, and raised them 

 to a considerable heat ; but they had no effect in dissolving 

 the crystal. 



* Vol. ii. plate 1. fig. 16. 



t Edinburgh Transactions, vol. xiv. plate 10. figs, 1, 2. 



